Whilst a graduate student, I studied wild white-crowned sparrows, Zonotrichia leucophrys, for my research project. This of course, meant that I had to go into the field to places where the birds rested for the night during their migration so I could capture them in mist nets early the next morning.

But which precautions did I take to ensure "my" birds' safety? I was always careful to remove birds as quickly as possible after they hit the net, to carefully disentangle those individuals that had managed to thoroughly entangle themselves into the net, and I was always alert to signs of stress in the netted birds; taking care to prioritize removal of those individuals that puffed out their feathers, panted, became motionless or closed their eyes.

Unfortunately, some members of the bird watching community were less than enamoured with my day job. Heated debates regarding the ethics and risks associated with mist netting wild birds sometimes occurred, debates that I tried to avoid since, as a young scientist, I was recruited to defend "all those scientists" even though I couldn't cite any peer-reviewed studies that rigorously analysed the risks to the birds.

Because there weren't any.

Meanwhile, my detractors told disgusting but unverifiable stories to support their position whilst I could only remain silent, making a mental note to myself to avoid this person in the future.

Mist netting is a capture method commonly used by scientists and their assistants who study wild birds and bats. But surprisingly, despite the sometimes very vocal controversy in the animal rights and bird watching communities surrounding the use of mist nets, a systematic study of the risks involved to the animals had never been published. It's possible that the reason is because mist netting has been in wide use since the 1950s -- long before most scientists who use this method were born. But regardless of the reason for this oversight, before scientists and birders can even have a reasoned debate regarding the ethics of using mist nets to capture wild birds and bats, we first need to answer the questions: How safe is mist netting? What are the risks to the study subjects associated with mist netting?

Instead of being purely academic questions designed to add another paper to one's CV (curriculum vitae; this is an academic's résumé), the risks associated with mist netting birds are a valid concern. This was the motivation to conduct this study in the first place: Erica Spotswood, a doctoral candidate at the University of California at Berkeley, wrote a research proposal to study the Gray-green Fruit Dove, Ptilinopus purpuratus, in French Polynesia. Her request was denied because she was unable to cite any studies that quantified the risks that mist netting posed to birds, which was how she proposed to capture her study birds.

"I was very surprised to find that no study of this kind existed, because mist netting has been around since the 1950s and is an extremely widely used and common technique for monitoring bird populations," said Ms Spotswood. So she decided to do something about this.

"In the United States at least one million new birds are banded [ringed] each year at several hundred bird observatories around the country," Ms Spotswood said. Most of those organisations keep detailed capture records.

That's a lot of data points.

Ms Spotswood gathered together a team of scientists from California and they approached 70 bird observatories and banding organisations in the United States and Canada to share their capture data so the team could learn more about the risks to birds associated with mist netting. Twenty-two organisations responded.

Analysis of the 620,997 reported captures revealed that injuries and death were rare occurrences (average injury: 0.59% ± 0.68; average mortality: 0.23% ± 0.15). Further, whilst the injury rate exceeded the mortality rate, even when combined, this "incident rate" was less than 1 percent for 18 of 22 organizations (the total combined incident rate was just 0.61% ± 0.66). To provide some context, organisations that mist net birds strive to keep their "incident rate" lower than 1 percent. Compare this to several published mammal studies that required leg traps [Blundell et al. 1999], snares or helicopter darting [Del Giudice et al. 2005] that reported mortality and injury rates above 1 percent.

"Using this enormous data set we could consider not only the rate of injuries, but could ask which species are most at risk and what kinds of incident are most common," said Ms Spotswood.

Five bird observatories volunteered to participate in additional studies by providing detailed records of their captures. These data, compiled during more than 20 years of research, included 345,752 captures of 188 species of birds from 31 taxonomic families. The team analysed these data to categorise and quantify the types of injuries and mortalities that mist-netted birds experienced thusly (figure 1):

Fig. 1. Common categories of (a) injury and (b) mortality associated with mist netting and handling. [NOTE: x-axis categories are not in the same order for both figures]

Although injuries and deaths were very rare, the researchers found that the most common types of injuries and mortalities stemmed from wing injuries, stress and cuts. Perhaps not surprisingly, they found that the most common sorts of injuries varied between different bird species. For example, spotted towhees and Allen's hummingbirds were more prone to stress, whereas western tanagers and American robins were more prone to wing strain. Based on these observations and preliminary data, the team predicted that species, body size, age, sex and the timing of capture could influence the likelihood of an incident.

To test this hypothesis, the team analysed the PRBO Conservation Science (formerly Point Reyes Bird Observatory) data set for time of day, individual mass, and total number of times a particular individual had been captured, and compared these data for male and female birds (figure 2):

Fig. 2. Predicted numbers of incidents per 1000 birds from the PRBO dataset for male and female birds as a function of (a) time of day, (b) body mass (mass values are individual body measurements for each capture), and (c) the capture number (total captures for an individual). Results were model averaged across three top-performing models in the 95% confidence set.

The researchers found that the predicted number of incidents increased slightly from 5:00 am to noon from 3 to 3.5 birds per 1000. But the number of incidents more than doubled from three to seven birds per 1000 as individual body mass increased and declined from four to less than one incident per 1000 as the capture number increased. So these data show that birds with a higher body mass that were caught for the first time in their lives around noon were most likely to experience either injury or death as a direct result of mist netting. Meanwhile, the data show that sex, age, month of capture (think: "migration") and mist net mesh size had little effect on the predicted number of incidents.

The team thought about these data and predicted there might be species-specific risks based on anatomy, physiology and behaviour. To identify which species might be most vulnerable to incident, the team selected the 36 most commonly captured species, each of which had been captured more than 2000 times (figure 3):

Fig. 3. Predicted percent of total incidents for eight types of incident for birds as a function of body size (mass values are species averages). Data are taken from five contributing organizations and include only incident records. Each model included species, organization, year, species × organization and year × organization random effects.

The authors found a correlation between tangling, body mass and stress levels, and the rate of incidents. Based on these data, the authors propose an entirely reasonable scenario where birds with lighter body mass can become more entangled in the nets, which then leads to longer extraction times. These prolonged periods of handling would certainly be responsible for increased risk of stress, and are probably also related to tangling-related injuries and wing strain in smaller birds.

However, the authors were not able to explain all their data.

"[W]e did not find obvious patterns in behaviour or anatomy that could explain why spotted towhees, Allen's hummingbirds, American robins and western tanagers were more prone to incident than other birds in the 36 species data set," the authors write.

Finally, the team returned to the long-term PRBO data and evaluated it to determine whether recapture rates or histories were different for birds that were released after an injury relative to those that did not sustain injuries (figure 4):

Fig. 4. Average recapture history over time for injured and uninjured birds from 12 species captured by PRBO between 1988 and 2008 and seven species captured by IBO between 1997 and 2008. Error bars represent 1 SE.

These data show that injured birds had higher rates of recapture than uninjured birds, probably because injured birds remained in the area to recover after release more often than did uninjured birds. However, the history of recapture over time was not different for injured and uninjured birds, suggesting that birds that are released when injured continue to survive in similar numbers to those that are released uninjured.

This study is valuable because it is a fundamentally important piece of work. It's remarkable that nothing has been done to assess the risk of mist netting before. But that said, this study confirms my own experiences with mist netting wild birds by showing that the rate of incidents (injuries and mortalities) are rare. It also serves to underscore those precautions I'd been taught to rigorously observe to safeguard the health of "my" mist netted birds:

1. vulnerable species and individuals captured for the first time should be prioritized. Further, banders/ringers should identify which species are most vulnerable at their own sites.

2. banders/ringers should pay attention to stress cues in netted birds such as panting, lethargy, raising of feathers and closed eyes. Further, banders/ringers should be particularly diligent to recognise stress cues in the smaller bird species.

3. banders/ringers should be especially alert to signs of wing strain and tangling in smaller birds and to potential internal injuries, leg injuries, cuts and predation in larger birds.

"What began as an inquiry for a permit application ended up evolving into something we feel will be of value to the scientific community," remarked Ms Spotswood.